AP Chemistry · AP Chemistry CED Unit 8 · 14 min read
1. Acid-Base Reactions and Neutralization Stoichiometry★★☆☆☆⏱ 4 min
Acid-base reactions are proton-transfer reactions under the Bronsted-Lowry model: an acid donates a proton, and a base accepts a proton. Neutralization reactions between acids and bases go to completion whenever a strong acid or strong base is involved, meaning you must always solve limiting reactant stoichiometry first before any equilibrium pH calculation.
HA + B \rightleftharpoons A^- + HB^+
Exam tip: Always complete the stoichiometry step before any equilibrium pH calculation when mixing acids and bases, even for buffer problems.
2. Buffer Composition and the Henderson-Hasselbalch Equation★★★☆☆⏱ 5 min
A buffer is a solution that resists large pH changes when small amounts of strong acid or base are added. Valid buffers contain appreciable amounts of a weak conjugate acid-base pair: either a weak acid plus its conjugate base (as a soluble salt), or a weak base plus its conjugate acid.
Exam tip: You can always use moles instead of concentrations in the Henderson-Hasselbalch equation, eliminating errors from forgetting to update total volume after mixing.
3. Buffer Capacity★★★☆☆⏱ 3 min
Buffer capacity is a measure of how much strong acid or strong base a buffer can absorb before pH changes by a large, unacceptable amount. It depends on two key factors: (1) total concentration of buffer components: higher total concentration gives higher buffer capacity, and (2) the ratio of conjugate base to weak acid: maximum buffer capacity occurs when $[A^-] = [HA]$, so $pH = pK_a$.
Buffers are considered effective for pH values within $\pm 1$ unit of the weak acid's $pK_a$. Outside this range, the ratio of components is more than 10:1, so adding a small amount of strong acid/base changes the ratio drastically, leading to a large pH change.
Exam tip: When asked to select the best buffer for a target pH, the weak acid with $pK_a$ closest to the target pH (within 1 unit) is always the correct choice, all else equal.
4. AP-Style Practice★★★★☆⏱ 2 min
Common Pitfalls
Why: Students memorize the equation and reach for it automatically regardless of the actual composition of the solution.
Why: Students forget that added strong acid/base reacts with buffer components to change the amount of each component.
Why: Students incorrectly assume any conjugate acid-base pair forms a buffer.
Why: Students misremember the derivation and reverse the terms.
Why: Students confuse buffer capacity (how much acid/base can be absorbed) with the current pH of the buffer.